The present invention relates to electronic ballasts for gas discharge lamps, and more particularly, to a single-switch-inverter-based electronic dimming ballast for a fluorescent lamp.
Electronic ballasts for fluorescent and other gas discharge lamps are well known. A typical topology for an electronic ballast uses a half-bridge inverter circuit containing two semiconductor switching devices such as two metal oxide semiconductor field effect transistors (MOSFETs). The inverter receives DC voltage provided by rectifying, and at least partly filtering, voltage from an AC supply, and delivers high frequency AC current, typically at a frequency of a few tens of kilohertz, to a fluorescent lamp.
There have recently been proposed ballasts using only a single semiconductor switching device in the inverter. By eliminating a second semiconductor switching device, these ballasts advantageously have reduced cost and reduced power dissipation. An example of such a ballast is described in commonly-assigned U.S. patent application Ser. No. 10/006,036, filed Dec. 5, 2001, entitled xe2x80x9cSingle Switch Electronic Dimming Ballastxe2x80x9d, which is herein incorporated by reference in its entirety.
One ballast described in that earlier application includes a transformer having a primary winding, a secondary winding, and a magnetizing inductance associated therewith. A semiconductor switching device is connected to the primary winding for applying a voltage across the primary winding. The secondary winding of the transformer supplies a current to a lamp through a resonant circuit, such as an LC resonant tank. When the switching device is conducting, a current is induced in the magnetizing inductance, and when the switching device is not conducting, a portion of the current in the magnetizing inductance flows to the resonant tank. A portion of the current flowing in the resonant tank flows to the fluorescent lamp.
A control circuit determines the operating frequency and duty cycle of the semiconductor switching device. The operating frequency is typically selected to be as close as possible to the resonant frequency of the tank circuit so as to achieve needed performance in terms of voltage gain, waveform smoothing, and ballast output impedance. However, if the operating frequency is set too close to the resonant frequency of the tank, then there are significant power losses due to circulating currents.
The maximum value of the magnetizing inductance of the transformer is limited by the maximum power that needs to be supplied to the lamp. The magnetizing inductance has hitherto been set as high as possible, however, so as to minimize currents through the switching device and the transformer. Excess current causes additional power dissipation that is wasteful and potentially harmful, because it is dissipated as heat in components that may be damaged, or have reduced service lives, if they are overheated. There is thus a clear and well-known motivation to keep excess current to a minimum.
The present invention is based in one aspect on the discovery that, with certain ballast topologies, when the inverter switch is non-conductive, the magnetizing inductance of the transformer interacts electrically with the resonant tank circuit. As a result, the effective resonant circuit then consists essentially of the original tank circuit, typically a tank capacitor and a tank inductor, plus the magnetizing inductance. The resulting combined circuit has a different, typically lower, resonant frequency than the original resonant tank circuit.
Based on this discovery, the performance of the ballast at low power outputs, when the switch is nonconductive (the xe2x80x9coff timexe2x80x9d) for a large proportion of the switching period, can be substantially improved by selecting the operating frequency of the ballast in relation to the resonant frequency of the effective resonant circuit including the magnetizing inductance. However, merely lowering the operating frequency can result in diminished ballast performance, especially at high power outputs. In order to avoid diminished performance at high power outputs, when the switch is conductive (the xe2x80x9con timexe2x80x9d) for a higher proportion of the switching period, the value of the magnetizing inductance is preferably reduced. The resonant frequency of the combined resonant circuit, including the magnetizing inductance, is thereby brought closer to the resonant frequency of the tank circuit, excluding the magnetizing inductance.
In one aspect, the invention provides an electronic ballast for discharge lamps, comprising a single-switch inverter including an inductor, preferably a magnetizing inductance, and a first resonant circuit, preferably an LC tank circuit, connected to the inverter output, wherein when the inverter switch is non-conductive the inverter inductor forms with the components of the first resonant circuit a second resonant circuit, wherein the operating frequency of the inverter is controlled to be at or below the resonant frequency of the first resonant circuit, and wherein the value of the magnetizing inductance is substantially lower than the maximum value that would permit the ballast to supply its maximum desired output power.
In another aspect, the invention provides an electronic ballast for a fluorescent lamp, comprising a single-switch inverter including an inductor, and a first resonant circuit having a first resonant frequency, wherein when the inverter switch is non-conductive, the inverter inductor combines with an inductance of the first resonant circuit, forming a second resonant circuit having a second resonant frequency lower than the first, wherein the operating frequency of the inverter is below the first resonant frequency, and wherein the operating frequency of the inverter is close to the second resonant frequency.
The operating frequency of the inverter is preferably closer to the second resonant frequency than to the first resonant frequency. Advantageously, the operating frequency of the inverter is no more than half as far from the second resonant frequency as it is from the first resonant frequency. Preferably, the operating frequency of the inverter is less than the second resonant frequency. Stated differently, the invention is an electronic ballast for fluorescent lamps comprising a single-switch inverter including an inductance and having an operating switching period; and a resonant circuit supplied by the inverter and having a first resonant period; wherein when the inverter switch is non-conductive, the inverter inductance interacts with the resonant circuit to define a second resonant period longer than the first resonant period; wherein the operating switching period of the inverter is longer than the first resonant period; and wherein the duration of the operating switching period of the inverter is close to the duration of the second resonant period. Preferably, the duration of the operating switching period of the inverter is closer to the duration of the second resonant period than to the duration of the first resonant period. Most preferably, the duration of the operating switching period of the inverter is no more than half as far from the duration of the second resonant period as it is from the duration of the first resonant period.
The operating frequency may be set between the two resonant frequencies so that the power consumption of the ballast when the ballast is operating under a xe2x80x9cno-loadxe2x80x9d condition is no greater than the power losses in the ballast when operating at full power.
It is possible to vary the operating frequency of the inverter, so as to be closer to the resonant frequency of the first resonant circuit when the duty cycle is high, and to be closer to the second resonant frequency when the duty cycle is low. This control method is not necessary for operation of the ballast, but may be advantageous for some applications, such as, for example, when driving small-diameter lamps. One advantage of the present invention is that it exploits the simplicity, and thus the low cost and high reliability, of a single-switch inverter.
The present invention is based in another aspect on the discovery that the circuits according to the invention make it possible to control the transition of conductivity of the switch in the inverter such that the switch experiences a zero current switching transition from a state of being non-conductive to a state of being conductive. By causing the switch to transition from a non-conductive state to a conductive state when the current in the magnetizing inductance is equal to the current in the tank inductor, or at a time when the current in a clamp circuit associated with the inverter is substantially zero, the switch will experience a zero current switching event. This zero current switching event is highly preferable in that the power loss associated with the switching event will be substantially reduced as compared with an otherwise similar ballast not using the zero current switching of the present invention.
The invention accordingly provides in one aspect a flyback inverter for a fluorescent lamp ballast that provides zero-current switching in continuous-conduction mode operation. In an especially preferred aspect of the invention, the ballast includes a tank circuit coupled to the output of the inverter, and, immediately before the switch closes, that is, before the switch transitions from a non-conductive state to a conductive state, the current flowing through the magnetizing inductance is substantially equal to the current flowing through the tank inductor. The inductive reactance of the two inductors forces these currents to keep flowing and prevents any immediate diversion of current through the switch or the clamp circuit.
In another aspect, the invention provides a single-switch inverter with a clamp circuit to limit the voltage across the magnetizing inductance when the switch is off, which is arranged so that the inverter switch is preferably turned on only at a time when the clamp circuit is not carrying a current. This is especially preferred in a circuit where ringing may cause intermittent operation of the clamp circuit for some time after the inverter switch is turned off.
In another aspect, a ballast constructed in accordance with the instant invention may comprise: an inverter connectable to a source of power for supplying a fluorescent lamp with a high-frequency current, the inverter including a single controllably conductive device for switching current in the inverter, a first inductor electrically connected to the controllably conductive device such that when the controllably conductive device is in a conducting state, the rate of change of the current in the first inductor is defined by the voltage applied to the inverter by the source of power; at least one second inductor electrically connected to the first inductor such that when the controllably conductive device is in a non-conducting state, at least a portion of the current in the first inductor flows through the at least one second inductor; and a control circuit arranged to cause the controllably conductive device to become conductive at a time such that the non-transient current through the controllably conductive device immediately after being caused to become conductive is substantially equal to the non-transient current through the controllably conductive device immediately before being caused to become conductive.